Method for the post-treatment of exhaust gases by subsequent injection of fuel in a diesel-type internal combustion engine provided with a pre-catalyst and a particle filter

ABSTRACT

A process is proposed for exhaust gas aftertreatment by afterinjection of fuel in a diesel internal combustion engine with a precatalyst and a particulate filter, in which process the afterinjection NE is performed as a function of the temperature in the area of the precatalyst T cat  and of the state of charging of the particulate filter. Since this process operates with especially high efficiency in heretofore critical short-distance operation of diesel internal combustion engines, the strict requirements of future legislation relating to exhaust gas emissions can immediately be met.

BACKGROUND OF THE INVENTION

This invention relates to a process of exhaust gas aftertreatment bysecondary injection of fuel in a diesel internal combustion engine withprecatalyst and particulate filter.

Use of a particulate filter to effectively reduce particulate emissionsby a diesel engine effectively will be unavoidable in the future.Because of low exhaust gas temperatures, the filter is constantlycharged with soot, something which in the extreme case can result inbreakdown of a vehicle, especially in the event of repeatedshort-distance operation.

Document EP 0 621 400 A 1 discloses an air compressing internalcombustion injection engine with exhaust gas aftertreatment equipment. Areducing agent may be fed to this internal combustion injection enginein order to reduce nitrogen oxides. The reducing agent is prepared inthe final stage of combustion downstream from the ignition OT bysecondary injection of fuel.

In addition, document DE 197 35 011 A1 discloses a process in whichexhaust gas temperature dependent and time dependent secondary fuelinjection is prepared for the purpose of nitrogen oxide reduction. Forthis purpose transition is made from a first secondary injection mode toa second secondary injection mode involving a secondary injection amounthigher than in the first mode.

Secondary injection or afterinjection of fuel in each instance servesthe purpose of raising the exhaust gas temperature by means of anexothermal reaction which takes place within a specific exhaust gastemperature range; however, temperature alone does not represent asuitable criterion for effective secondary injection of fuel.

SUMMARY OF THE INVENTION

With this state of the art taken as a basis, it is the object of thisinvention to prepare an improved process for exhaust gas aftertreatmentin which practical use of a particulate filter is made possible,especially in the event of frequent use in short-distance operation ofdiesel internal combustion engines, and so represents further advance incompliance with future exhaust gas emission standards as well as anopportunity for securing tax incentives for its development.

This object is attained by means of a process having the featuresspecified in claim 1. Since afterinjection is performed both as afunction of the temperature in the area of the precatalyst T_(cat) nearthe engine and as a function of the state of charging of the particulatefilter, both the activation and the amount of afterinjection NE for eachload point may be ideally selected so that rapid heating of theparticulate filter is ensured by exothermicity at the precatalyst,without significant penetration of CO or HC. As a result of theconversion of HC and CO which has already taken place at theprecatalyst, NO may subsequently be oxidized to NO₂ with a high yield,which is subsequently required for oxidation of the soot in theparticulate filter.

BRIEF DESCRIPTION OF THE DRAWING

Reference is made to the accompanying drawing in which FIG. 1illustrates in simplified form the variation in temperature in the areaof the T_(cat) over time t.

DETAILED DESCRIPTION OF THE INVENTION

The afterinjection NE is activated when the temperature in the area ofthe precatalyst T_(cat) exceeds a light-off temperature T_(LO) of around160° C., since it is only above this light-off temperature that theconversion rate for unburnt hydrocarbons amounts to at least 50% andthat further elevation of the temperature can take place as a result ofafterinjection NE of additional fuel, so that regeneration of theparticulate filter is introduced. The temperature in the area of theprecatalyst T_(cat) is measured by a thermocouple downstream from theprecatalyst.

Afterinjection NE, on the other hand, is deactivated when thetemperature in the area of the precatalyst T_(cat) exceeds an upperregeneration temperature T_(RO) of around 280° C. In addition, theafterinjection NE is reactivated when the temperature in the area of theprecatalyst T_(cat) falls below a lower regeneration temperature T _(RU)of around 230° C. A sufficiently large regeneration temperature rangeΔT_(R) of about 50° C. is thereby determined within which oxidation ofsoot by NO₂ is ensured.

Provision is additionally made such that afterinjection NE orregeneration of the particulate filter does not exceed a specific periodof time t_(limit). This period of time t_(limit) amounts, for example,to approximately 20 seconds and represents a sort of “emergencyshutdown” if the precatalyst fails to start up.

For the sake of safety, provision is also made such that, if theprecatalyst fails to start up, repeated afterinjection NE may take placeonly after a specific time delay t_(pause). It is proposed that thistime delay t_(pause) amount to approximately 30 seconds and that itpromote the possibility of subsequent repeated startup (“light-on”) ofthe precatalyst.

It is advantageous for the state of charging of the particulate filterto be indicated by means of a counter Z and for the afterinjection NE tobe permitted after a specific counter reading Z_(reg) is displayed whichcorresponds to an assigned charging state. In this way the chargingstate of the particulate filter may be displayed in a simple manner bymeans of a computer or modified and used as a criterion forafterinjection NE, and thus for regeneration of the particulate filter.

The counter Z reading is increased if the average regenerationtemperature T_(RM) between the upper regeneration temperature T_(RO) andthe lower regeneration temperature T_(RU) has not been reached within aspecific interval t_(start) after cold starting of the diesel internalcombustion engine, since otherwise soot accumulation takes place withinthe particulate filter so that a higher state of charging is present.

In addition, the reading of the counter Z increases in the event ofextreme short-distance operation. Such extreme short-distance operationmay even make additional measures necessary, ones such as an electricdevice for heating the precatalyst, restriction of intake air flow, orshifting of the injection times in the “late” direction in conjunctionwith higher engine speed. Preference is then given to employment ofthese measures beginning with the next startup of the diesel internalcombustion engine and may reduce the reading of the counter Z by afreely selectable amount.

In addition, the reading of the counter Z is lowered gradually by aspecific amount if the mean generation temperature T _(RM) between theupper regeneration temperature T_(RO) and the lower regenerationtemperature T_(RU) is expected, since, when this sum t_(sum) ofestablished time intervals t_(int) is present, burning away of the sootensues, so that the state of charging of the particular filter iscorrespondingly reduced. If decrease in the rate of reduction of thecounter Z reading ensues with allowance made for hte reaction kineticsas a function of the temperature, the operation of the counter Z withallowance made for this temperature dependence is characterized byespecially high accuracy.

In addition, it is advantageous, in order to determine the state ofcharging of the particulate filter, also to monitor the back pressure inthe exhaust gas system, since, although the back pressure does not ofitself represent a suitable criterion for a specific charging statebecause any holes possibly present in the layer of soot result in arelatively low back pressure falsely indicating too low a chargingstate, additional certainty in determining the charging state of theparticulate filter can nevertheless be provided by monitoring the backpressure.

Reference is made to the accompanying figure, which presents insimplified form the variation in temperature in the area of the T_(cat)over time t, in order to illustrate the foregoing explanations.

1. A process for exhaust gas aftertreatment by afterinjection of fuel ina diesel internal injection with a precatalyst and a particulate filter,the afterinjection NE being performed as a function of the temperaturein the area of the precatalyst T_(cat) and the state of charging of theparticulate filter, wherein the state of charging of the particularfilter is indicated by means of a counter Z and the afterinjection NE ispermitted only after a specific counter reading Z_(reg) corresponding toa prescribed state of charging has been reached, and the level of thecounter Z is increased if the mean regeneration temperature T_(RM)situated between the upper regeneration temperature T_(RO) and the lowerregeneration temperature T_(RU) is not reached within a specific timeperiod t_(start) after a cold start of the diesel internal combustionengine.
 2. The process as defined by claim 1, wherein the counter levelis progressively lowered when the mean regeneration temperature T_(RM)situated between the upper regeneration temperature T_(RO) and the lowerregeneration temperature T_(RU) is exceeded for a specific sum of timet_(sum) of established time intervals t_(int).
 3. A process for exhaustgas aftertreatment by afterinjection of fuel in a diesel internalinjection with a precatalyst and a particulate filter, theafterinjection NE being performed as a function of the temperature inthe area of the precatalyst T_(cat) and the state of charging of theparticulate filter, wherein the state of charging of the particularfilter is indicated by means of a counter Z and the afterinjection NE ispermitted only after a specific counter reading Z_(reg) corresponding toa prescribed state of charging has been reached, and the level of thecounter Z is increased in the event of extreme short-distance operation.4. The process as defined by claim 3, wherein the counter level isprogressively lowered when the mean regeneration temperature T_(RM)situated between the upper regeneration temperature T_(RO) and the lowerregeneration temperature T_(RU) is exceeded for a specific sum of timet_(sum) of established time intervals t_(int).
 5. A process as definedfor exhaust gas aftertreatment by afterinjection of fuel in a dieselinternal injection with a precatalyst and a particulate filter, theafterinjection NE being performed as a function of the temperature inthe area of the precatalyst T_(cat) and the state of charging of theparticulate filter, wherein the state of charging of the particularfilter is indicated by means of a counter Z and the afterinjection NE ispermitted only after a specific counter reading Z_(reg) corresponding toa prescribed state of charging has been reached, and the counter levelis progressively lowered when the mean regeneration temperature T_(RM)situated between the upper regeneration temperature T_(RO) and the lowerregeneration temperature T_(RU) is exceeded for a specific sum of timet_(sum) of established time intervals t_(int).
 6. The process as definedin claim 5, wherein the level of the counter Z is lowered as a functionof temperature.
 7. The process as defined by claim 6, wherein theafterinjection NE must not exceed a specific period of time t_(limit).8. The process as defined by claim 7, wherein repeated afterinjection NEmay be performed only after expiration of a specific period of timet_(pause).